On Thu, Nov 19, 2015 at 01:06:17PM +0100, Paolo Bonzini wrote: > > > On 19/11/2015 13:01, Michael S. Tsirkin wrote: > >> > You know... I took a look at this, and yes, i could simply hook up > >> > emulation into memory region handlers. And everything that > >> > expects KVM eventfd binding will magically start working, probably > >> > rendering some bypass code obsolete. > >> > I have only one concern against this. qemu is a large piece of > >> > software, consisting of lots of components. I cannot test absolutely > >> > everything in every configuration. I suggest, old code was written with > >> > the assumption that if memory_region_add_eventfd() works, we > >> > are really using KVM acceleration. If we break this assumption, how much > >> > code will mysteriously misbehave instead of throwing > > First of all, memory_region_add_eventfd simply exits on failure. > > It seems unlikely you will break something which isn't already > > broken. > > > > Further: > > > > $ git grep memory_region_add_eventfd > > hw/misc/ivshmem.c: memory_region_add_eventfd(&s->ivshmem_mmio, > > hw/misc/pci-testdev.c: memory_region_add_eventfd(test->mr, > > hw/virtio/virtio-mmio.c: memory_region_add_eventfd(&proxy->iomem, > > VIRTIO_MMIO_QUEUENOTIFY, 4, > > hw/virtio/virtio-pci.c: memory_region_add_eventfd(modern_mr, > > modern_addr, 0, > > hw/virtio/virtio-pci.c: memory_region_add_eventfd(modern_mr, > > modern_addr, 2, > > hw/virtio/virtio-pci.c: > > memory_region_add_eventfd(modern_notify_mr, 0, 2, > > hw/virtio/virtio-pci.c: memory_region_add_eventfd(legacy_mr, > > legacy_addr, 2, > > include/exec/memory.h: * memory_region_add_eventfd: Request an eventfd to > > be triggered when a word > > include/exec/memory.h:void memory_region_add_eventfd(MemoryRegion *mr, > > include/exec/memory.h: * memory_region_add_eventfd() call. > > memory.c:void memory_region_add_eventfd(MemoryRegion *mr, > > > > Not such a bit deal to audit all call sites, is it? > > > > Cc memory API maintainer for an opinion. > > > > Paolo, do you see anything wrong with making > > memory_region_add_eventfd work (slowly) without kvm ioeventfd support? > > > > Sure, it's even been on the todo list for a while. Stefan Hajnoczi had > a patch, the only ugly thing was that it slowed down a little all > non-ioeventfd accesses, but I guess that's okay because it's probably > not measurable. > > http://lists.nongnu.org/archive/html/qemu-devel/2015-07/msg04710.html > > Paolo > > Paolo
Maybe it can be imporved to probe the kernel and only take action if kvm does not support ioeventfd. -- MST
